The initial configuration refactoring already gave significant benefits
with my use case. On another note, Commands should know and ask for
configuration variables. We could, as we refactor, add the ability
for commands to tell what configuration variables knows about and
their types.
This way, completers can be used too when using `config` command if we
also add a sub command that config could set variables to.
Commands stating the config variables they know about will allow us
to implement it in `help` and display them.
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
